How to load modules in Java

Question

I'm building a server that loads modules. Each module is a .jar file. Inside the jar there's all the classes that the module needs. The server needs to read the jar, find the main class (it doesnt have the main method on it is just the class that makes the module work but not as a different program), build an object of that class and store it in a vector of modules so that it can acces to a specific module depending on the job to be done.

How can I do that?

As far as the server, it is listening and receiving request, but there's no modules to delegate the job to, since my vector of modules is empty.

Answer 1

Here is a network class loader example from Sun. This should cover everything you need.

Try this syntax once you get the URL with you original URLClassLoader

JAR URL

The syntax of a JAR URL is:

jar:<url>!/{entry}

for example:

jar:http://www.foo.com/bar/baz.jar!/COM/foo/Quux.class
jar:file:/export/home/faculty/stansif/public_html/java/applets/image/view.jar!/image.gif

Accessing resources

final java.net.URL url = X.class.getResource ("/dir/image.png");
final ImageIcon icon = new ImageIcon (url);

Also look at this URL http://www.javaworld.com/javaworld/javatips/jw-javatip70.html

Answer 2

You should look at existing frameworks, including a lightweight OSGi container. However, the general approach that is taken is to create a classloader for each module, and call into each module using Reflection or a common API -- Runnable and Callable being two good candidates.

Answer 3

You should use Java's ServiceLoader class,

http://java.sun.com/javase/6/docs/api/java/util/ServiceLoader.html

This requires your JAR has a provider configuration file in META-INF/services directory. This is the standard built-in way to load modules in Java. If you need more robust module loading, you might want look at OSGi.